There are various applications in which an input signal is applied to the primary of a transformer whose secondary is coupled to an output circuit that produces a signal having a specified relationship to the input signal. An example of such a circuit is a current transformer circuit employed in applications such as so-called protective relaying for power transmission lines.
AC power transmission lines are often protected by protective relaying systems which operate upon occurrence of a fault to trip circuit breakers that protect the transmission line from damage and isolate the faulted portion of transmission line from the rest of an overall transmission system. Typically, the section of transmission line to be protected extends between terminals called local and remote terminals, and substantially identical protective subsystems are located at the remote and the local terminals. Current on the transmission line (generally, on individual phases thereof) is sensed at both the local and the remote terminals, and information concerning the current is transmitted over a communications channel from the remote terminal to the local terminal, and vice versa. Each subsystem includes means for comparing the local and remote current readings and for generating trip control signals as a function of the comparison. The trip control signals operate, under certain conditions, to trip circuit breakers at the respective locations when the subsystems detect a condition that indicates an internal fault; i.e., a fault within the protected section of transmission line. A prior art protective relaying system is disclosed, for example, in U.S. Pat. No. 4,939,617, assigned to the same assignee as the present application.
The accurate sensing of current (typically, for each individual phase of the transmission line) is essential to proper performance of the protective relaying system. Each current transformer circuit operates to produce an output voltage that is proportional to the current in the current transformer primary winding. The output voltages are then used in subsequent processing for determination of the absence or presence of a fault, as previously described. The current transformers are required to operate over a wide range of current magnitudes and, when a fault occurs, a sharp exponentially decaying DC offset can result. If the transformer core isn't large enough to handle the flux that results from the offset, it will saturate. A substantial recovery time may then be needed before the transformer will again operate properly, and this could result in serious consequences in an application such as protective relaying, since the system will be effectively "blinded" until the transformer recovers. [As an example, in differential type protective relaying, should saturation occur at one station and not the other, or at both stations, an inaccurate signal comparison will be made and could result in a false trip in a thru-fault situation, or a failure to trip upon a line fault.] In extreme cases, the transformer could be permanently disabled.
The use of a large core can prevent the stated problem, but the attendant size, weight and cost are generally undesirable. Air gaps can make the core more resistant to saturation, but generally do not permit core size reduction. An approach to the core saturation problem which permits use of a relatively small core in certain applications has been to employ a flux cancellation circuit. In this approach, the transformer is provided with a primary winding, a sense winding, and a feedback winding. A sense winding circuit is responsive to the flux sensed by the sense winding and applies a signal to the feedback winding that tends to reduce the flux sensed at the sense winding. An output signal depends on the signal that is applied to the feedback winding and a feedback circuit feeds back the output signal to the sense winding circuit to provide stability. The feedback circuit includes a low-pass filter, which comprises an RC circuit. The circuit operates to reduce the flux in the transformer, but suffers a serious problem, as follows: A substantial current offset at the primary will cause a DC component in the feedback circuit that tends to saturate the input amplifier of the sense winding circuit. When this happens, control of the feedback loop is lost, the flux cancellation is not effective, and the transformer can saturate.
It is among the objects of the present invention to provide an improved circuit and method for preventing saturation of the core of a transformer when there are substantial offsets at the transformer primary.